1. Field of the Invention
The present invention relates generally to capacitive touch switches and, more particularly, to a shielding arrangement for reducing the sensitivity of a capacitive touch switch to stimuli other than an actual human touch.
2. Description of the Prior Art
Touch switches have certain advantages when employed as user control inputs in appliances and the like. Their general advantages include making it possible to have a smooth control panel surface for good appearance and easy cleaning, eliminating reliability problems caused by mechanically movable switch contacts, and permitting convenient interface to circuitry in so-called "electronic" appliances. For example, touch switches have been applied to both conventional electric ranges and countertop microwave ovens, in both cases in combination with circuitry employing digital logic techniques.
Touch switches are of two general types: resistance bridging and capacitive. In the resistance bridging type of touch switch, a touch element includes at least two electrodes. When a person touches the touch element, skin resistance across the two electrodes is sensed by associated circuitry which produces a control output. The capacitive type of touch switch, on the other hand, does not depend upon skin resistance, but rather depends on capacitive effects associated with the human body. The present invention is applicable to the capacitive type of touch switch, which type is more fully described below.
Capacitive touch switch devices generally include a panel or substrate in the form of a dielectric sheet element, for example a glass panel. The panel is also known as a "crystal." A conductive touch pad is disposed upon the front side of the panel. On the rear side of the panel, just opposite the touch pad, there is disposed at least a conductive sensing pad. The front touch pad and the rear sensing pad thereby form the two plates of a capacitor, with the panel serving as the dielectric between the two capacitor plates.
There are two specific types of prior art capacitive touch switch systems to which the present invention is applicable. The first is the "60 Hz pickup" type, which depends upon an external signal being inserted into the system through the touch pad. The second is the "capacitive attenuator" type, which shunts an internally-generated signal to ground when a person touches the touch pad. Both of these specific types are described in greater detail below under the heading, "Description of the Preferred Embodiments."
One disadvantage associated with the capacitive touch switches of either of the above types is undesired response to stimuli other than the actual touch of a human finger. This may occur, for example, when the system responds in apparently random fashion to a change in external electric fields. Another example of undesired response is a response to a human finger or other object coming close but not actually touching the touch pad. Desirably, a touch switch should function only as a touch switch, and not as a proximity detector. Of course, this latter effect cannot be entirely eliminated because, since the touch switch is a capacitive device, when an object such as a finger is close to the touch plate, for example, less than one-quarter inch, substantial coupling can occur. This effect can be minimized but not completely eliminated.
One approach to minimizing this undesired response is providing a gounded shield generally over the front of the panel, with openings in the shield exposing the touch pad beneath. Such a shield does effectively reduce the response of the touch switch system to undesired stimuli and can effectively prevent external stray capacitances from triggering the system. It is particularly effective in preventing undesirable response when a person's finger is near, but not too near, the touch pad; for example, two inches away as compared to one-quarter inch away. In such case, due to the relative nearness of the grounded shield as compared to the finger, capacitance associated with the finger produces a relatively minor effect on the circuit.
However, such a grounded shield disposed generally on the front side of the panel is not without its disadvantages. One example is awkward construction, particularly with regard to indicia and decorative patterns which may be placed on the front of the panel. Further, the use of a metallic shield over the front of a glass panel, even though the shield material may be rather thin, detracts from aesthetic qualities otherwise possible with such a smooth glass panel.
The present invention provides shielding for such capacitive touch switches in an improved manner which does not require a conductive shield placed over the front of a panel with its attendant disadvantages.